On premise telephone loop tester

ABSTRACT

The instant invention relates to a device that permits a telephone line (loop) to be tested from a telephone central office, thereby eliminating the expense resulting from a visit by a repairman to a subscriber&#39;s premises. The disclosed device is responsive to an audio tone of a predetermined frequency, is located on a subscriber&#39;s premises and is activated by an audio tone of frequency within the pass band of the telephone circuit, this tone being transmitted from a central office. A tuned detector, located within the device, upon receiving said predetermined tone, actuates a timing circuit and relay circuit that places a circuit across the tip and ring of a telephone line. The circuit simulates the electrical condition of a telephone instrument in the &#34;off hook&#34; position and causes central office line equipment to operate, if the telephone loop is in an operable state, thereby indicating that the telephone loop is complete from the central office to the subscriber&#39;s premises.

BACKGROUND OF THE INVENTION

There are many existing telephone loops that, due to their length,difficulty of access, or circuit makeup, are expensive to maintain. Itis quite often the case, when a repairman visits a subscriber premisesfollowing a trouble report, that it is found that the telephone loopetests out satisfactorily and the compliant was caused by no one toanswer the telephone, was due to a temporary equipment overload, or thetelephone equipment, other than the loop itself, on the subscriber'spremises was at fault. Thus, the repairman has made a trip to an extremeend of a telephone exchange for naught. In recent days, it is becomingmore and more prevalent for subscribers to install and maintain theirown on premise equipment, e.g., TWX, PBX, telephone equipment (handsets) and the like. Thus, reports of "telephone" trouble by a subscriberto a telephone company are beginning to be mixed bag, i.e., troublesregarding equipment to the telephone company is responsible for (mainlythe telephone loop) and troubles that the subscriber is responsible for(on premise equipment). The hereinafter disclosed invention allowstelephone company personnel to make the distinction as to where thereported trouble is and whose responsibility it is, without a repairmanleaving the maintenance depot.

SUMMARY OF THE INVENTION

The instant invention, a telephone loop tester, is a device that permitsa telephone line to be tested (whether it is operable or not) from atelephone central office without the expense of a repairman`s visit to asubscriber's premises. Such a device is especially valuable when atelephone loop consists of a carrier system because conventionalcapacitor charge tests techniques are inapplicable. Additionally, thetelephone loop tester, as hereinafter disclosed, is also applicable tovoice frequency circuits as well as circuits involving carrier.

When a subscriber reports trouble, telephone company personnel cantransmit an audio tone of a frequency, within a telephone circuit passband from a central office towards a subscriber's premises. This toneactivates the invention (telephone loop tester), which is on thesubscriber's premises. The on premise telephone loop tester contains atuned detector, which upon receiving a tone, activates a timing circuit.This timing circuit, in turn, operates a relay circuit, which places acircuit across the plus and minus (tip and ring) sides of the telephoneline. It is this circuit that simulates the electrical condition of atelephone instrument in the "off hook" position. This circuit is heldfor a predetermined length of time by a timer, and causes the centraloffice line equipment to operate, thereby indicating to personnel in thecentral office that the telephone loop (circuit) is complete from thecentral office to the subscriber's premises and return.

Power of operation of the detector, timer and relay circuit is suppliedby a nickel candium rechargeable battery, this battery being tricklecharged from the telephone line, and generally located at thesubscriber's premises. Alternatively, the detector, timer and relaycircuit can be powered from commercial power available at the subscriberend or other means that are well known.

The most practical frequency to use for activating the detector in theon premise telephone loop tester is 1,000 hz., available in mosttelephone central offices as a milliwatt supply; however, any frequencywithin the pass band of the telephone circuit may be used. In fact,several units, each tuned to a different frequency, may be used on thesame line, if ever required, such as a party line application or incases where the invention is used for telemetering. In order to preventactuation of the detector by voice energy, a diode switch is provided todisconnect the detector of the loop tester from the telephone circuitwhen the telephone circuit is in use.

BRIEF DESCRIPTION OF DRAWINGS

For the purpose of illustration of the instant invention, there is shownin the accompanying drawing, a circuit which makes up the on premisetelephone loop tester. It is to be understood, however, that thisdrawing is for the purpose of example only and the invention is notlimited thereto.

FIG. 1 is a schematic diagram of an on premise telephone loop testercircuit showing a detector, timer and relay circuit along with a powersource rechargeable from the telephone line itself; and

FIG. 2 is a schematic diagram of a telephone transmission systemcontaining the telephone loop tester circuit shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The on premise telephone loop tester is generally indicated at 26 inFIG. 2.

Making reference to FIG. 1, all transistors in the idle condition arecut off, i.e., not conducting; relay RY1 is released and the battery isbeing trickle charged by a current flowing in a path from the plus sideof the telephone line 1 to the minus terminal side of the line. Seeelements 1 and 2 as well as the designations tip and ring in FIG. 1.Also see U.S. Pat. No. 3,510,584 for disclosure regarding tricklecharging a battery from a telephone line, said disclosure hereinincorporated by express reference.

Coil L1 and capacitor C2 are tuned to a predetermined frequency, forexample 1,000 hz. Capacitor C1 is used to provide for A.C. coupling andD.C. blocking. Resistor R11 is selected in value to provide a properamount of trickle charge to battery B1. Resistor R3 has a value so thattransistor Q1 will present a high impedance to the tuned circuit (Coil 1and C2).

When 1,000 hz, the previously mentioned tone, is transmitted from thecentral office 20 (See Fig. 2) over the telephone pair 22 (See FIG. 2)it is received at the plus minus terminals (tip and ring). This signalthen flows through a path comprising resistor R1, diode D1, capacitorC1, the L1-C2 coil capacitor combination to the negative side of theline (ring). Since coil capacitor combination L1, C2 is tuned to thisfrequency, a voltage is developed across them, which, on positive peaksof the formentioned tone, will cause transistor Q1 to conduct. On eachpulse, a collector of Q1 is made negative due to the voltage dropoccurring across R2. This negative voltage is applied to the base oftransistor Q2 causing it to become conductive. In this electrical state,capacitor C3 causes the base of transistor Q2 to remain negative betweenpulses and effectively acts as a filter. While transistor Q2 isconducting, its collector becomes positive and through resistor R5,causes transistor Q3 to conduct.

When transistor Q3 conducts, capacitor C4 is being charged, therebysupplying a voltage across current limiting resistor R6. This voltage ishigh enough to cause zener diode D2 to conduct, placing the base oftransistor Q4 at a negative potential, thus causing transistor Q4 toconduct. The base of transistor Q4 remains negative for a length of timedetermined by the values of capacitor C4, resistor R7, diode D2 andresistor R8, such selection of values obviously being within the purviewof one of only ordinary skill in the art. When transistor Q4 conducts,the collector becomes positive which, when applied to the base oftransistor Q5 through resistor F10, causes transistor Q5 to conduct.This completes the path for the winding of relay RY1, thereby causingthe contacts of RY1 to close.

When the contacts of relay RY1 close, resistor R1 is thereby placedacross the ± (tip and ring) sides of the line causing a high current toflow over the talking pair. This high current flow causes an "off hook"signal to be sent towards the central office. This "off hook" signalremains a length of time determined by the time constants in thepreviously described circuitry. At the same instant that relay RY1contacts close, D1 becomes back biased and therefore nonconducting.Consequently, the tone input is removed from the detector. After thetone is removed, each transistor, beginning with transistor Q1, returnsto the nonconducting state. Q5 will remain in a conducting state for aperiod of time primarily determined by the RC time constant of C4-R7.Under normal conditions (non-testing) a telephone 24 (See FIG. 2), whichis connected across the tip and ring (±) side of the telephone linedefined by the telephone pair 22, is placed in the "off hook" condition,the voltage across tip and ring sides of the line drops to a point,below that of an unterminated loop condition, to where diode D1 is backbiased and therefore nonconducting. Under this condition, the detectoris disconnected from the line and cannot be operated by speech energyduring a conversation. Of course, it is possible to replace relay RY1with equivalents, such as a transistor. Furthermore, battery B1 can bereplaced by a power source at, on or near that of the subscriber'spremises.

The preceding disclosure has addressed itself to a situation where thereis no open or short in the telephone loop. However, if there is an"open" with the telephone loop, the "off hook" signal is not received atthe central office after the tone is transmitted therefrom because the"open" prevents the tone from completing its required electrical path.Obviously, if no tone is received at the on premise detector, it is notactivated. When this situation exists, telephone company personnel knowthat trouble, whatever the nature or cause, is in the telephone loop andnot in that equipment on the subscriber's premises. Other loop faults,i.e., "short," balance to ground, etc., are readily determined byexisting telephone C0 testing routines. The invention described hereinis ideally suited to determining those faults which degrade transmissionwithout showing a definite DC fault, and more especially to carrierderived circuits to which routine DC testing techniques are notapplicable. In the situation where a telephone company is responsiblefor both telephone loop and on premise equipment, the instant inventionpermits repairmen to eliminate either the loop or the equipment, thuspermitting these repairmen to concentrate their efforts on that elementof a telephone circuit that is causing trouble.

I claim:
 1. A telephone loop tester comprising first and secondconductors adapted to be electrically connected by a talking pair ofconductors of a telephone transmisson line to a central office in atelephone transmission system that has a telephone set connected to saidcentral office by said transmission line, and circuit means connected tosaid first and second conductors and responsive to the application of apredetermined, central office-transmitted signal to said first andsecond conductors for electrically simulating an off hook condition ofsaid telephone set without disconnecting said telephone set from saidcentral office by completing a current conducting path between saidtalking pair of conductors at a location that is remote from saidcentral office.[...]..Iadd., said predetermined signal having apre-selected frequency, and means for rendering said circuit meansnon-responsive to signals having said pre-selected frequency in responseto the transfer of said telephone set to its off hook state. .Iaddend..[.2. The telephone loop tester defined in claim 1 wherein saidpredetermined signal has a pre-selected frequency and wherein means areprovided for rendering said circuit means non-responsive to signalshaving said pre-selected frequency in response to the transfer of saidtelephone set to its off hook state..].
 3. A telephone loop testercomprising .Iadd.tuned .Iaddend.detecting means adapted to be connectedto a central office by a talking pair of conductors of a telephonetransmission line in a telephone system that has a telephone setconnected to said central office by said transmission line, saiddetecting means being operative to detect a signal of pre-selectedfrequency that is transmitted by said talking pair of conductors fromsaid central office .[.and.]. means responsive to the detection of saidsignal of pre-selected frequency by said detecting means forelectrically simulating an off hook condition of said telephone setremotely from said central office to provide an off hook signal whilesaid telephone set is in its on hook state and without disconnectingsaid telephone set from said central office.[...]..Iadd., said means forsimulating said off hook condition of said telephone set comprising acircuit rendered effective by said detecting means upon detecting saidsignal of pre-selected frequency for completing a current-conductingcircuit path that bridges said conductors in parallel relation with saidtelephone set and means rendered effective by the completion of saidcurrent-conducting circuit path for preventing application of adetectable signal to said detecting means while said current-conductingcircuit path is completed. .Iaddend. .[.4. The telephone loop testerdefined in claim 3 wherein said means for simulating said off hookcondition of said telephone set comprises a circuit rendered effectiveby said detecting means upon detecting said signal of pre-selectedfrequency for completing a current-conducting circuit path that bridgessaid conductors in parallel relation with said telephone set..]. .[.5.The telephone loop tester defined in claim 4 comprising means renderedeffective by the completion of said current-conducting circuit path forpreventing application of a detectable signal to said detecting meanswhile said current-conducting circuit path is completed..]. .[.6. Thetelephone loop tester defined in claim 4 comprising a unidirectionalcurrent-conducting device connected to said detecting means for blockingapplication of detectable signals to said detecting means whenever saidtelephone set is in its off hook state..].
 7. The telephone loop testerdefined in claim .[.6.]. .Iadd.31 .Iaddend.wherein said unidirectionalcurrent-conducting device and said detecting means are connected inseries, and wherein said means for completing said current-conductingcircuit path comprises switch means in parallel with said detectingmeans and said device.
 8. The telephone loop tester defined in claim.[.4.]. .Iadd.3 .Iaddend.wherein said circuit rendered effective by saiddetecting means comprises means for disabling said .[.circuit.]..Iadd.current .Iaddend.path after a pre-selected time delay. .[.9. In atelephone transmission system having a subscriber's telephone set and atalking pair of conductors connecting said telephone set to a centraloffice, the improvement comprising means connected to said conductorsremotely from said central office and responsive to a centraloffice-transmitted signal of pre-selected frequency for electricallysimulating an off hook condition of said telephone set by causing an offhook signal to be sent back to said central office as if said telephoneset were actually transferred to its off hook state and withoutdisconnecting said telephone set from said central office..]. .[.10. Ina telephone transmission system, a subscriber's telephone instrument, atwo conductor transmission line connecting said telephone instrument toa central office, and a telephone loop testing circuit electricallyconnected to the conductors of said transmission line remotely from saidcentral office and comprising means for detecting a centraloffice-transmitted signal of pre-selected frequency when said telephoneinstrument is in its on hook state, and means responsive to thedetection of said signal of pre-selected frequency by said detectingmeans for completing a circuit between said conductors in parallel withsaid telephone instrument while said telephone instrument is in its onhook state and without disconnecting said telephone instrument from saidcentral office to provide an off hook signal as if said telephoneinstrument were actually transferred to its off hook state..]. .[.11. Atelephone loop tester for a telephone transmission system that has asubscriber's telephone instrument connected by a two-conductortransmission line to central office equipment at a central office, saidtelephone loop tester comprising first and second conductors adapted tobe connected to the conductors of said transmission line remotely fromsaid central office and circuit means connected to aid first and secondconductors and responsive to a pre-selected central office-transmittedsignal for simulating an off hook condition of said telephone instrumentto provide an off hook signal that is capable of operating said centraloffice equipment as if said telephone instrument were actuallytransferred to its off hook state without disconnecting said telephoneinstrument from said central office and while said telephone instrumentis in its on hook state, the failure of said central office equipment tooperate upon transmittal of said pre-selected signal providing anindication that a fault or trouble may exist in said line..]. .[.12. Ina telephone transmission system having a subscriber's telephoneinstrument and a talking pair of conductors connecting said telephoneinstrument to central office line equipment at a central office, theimprovement comprising a circuit connected to said conductors remotelyfrom said central office and responsive to a central office-transmittedsignal of pre-selected frequency for simulating an off hook condition ofsaid telephone instrument to provide an off hook signal that is capableof operating said central office equipment as if said telephoneinstrument were actually transferred to its off hook state withoutdisconnecting said telephone instrument from said central office andwhile said telephone instrument is in its on hook state..].
 13. In atelephone transmission system, a subscriber's telephone instrument and atalking pair of conductors for connecting said telephone instrument to acentral office, said conductors being adapted to be connected to centraloffice-located equipment that is capable of being operated by the offhook signal that is produced by transference of said telephoneinstrument to its off hook state, and a telephone loop tester connectedto said conductors remotely from said central office and comprisingmeans for detecting a central office-transmitted signal of pre-selectedfrequency and a circuit responsive to the detection of said centraloffice-transmitted signal by said detecting means for completing acurrent-conducting circuit path that bridges said conductors in parallelwith said telephone instrument to provide an off hook signal that iscapable of operating said equipment as if said telephone instrument wereactually transferred to its off hook state without disconnecting saidtelephone instrument from said central office and while said telephoneinstrument is in its on hook state.[...]..Iadd., said telephone looptester further comprising means in series circuit relationship with saiddetecting means and rendered effective by the transfer of said telephoneinstrument to its off hook state to block application of detectablesignals to said detecting means and thereby prevent the detection of acentral office-transmitted signal of said pre-selected frequencywhenever said telephone instrument is in its off hook state. .Iaddend..[.14. The telephone transmission system defined in claim 13 whereinsaid telephone loop tester further comprises means in series circuitrelationship with said detecting means and rendered effective by thetransfer of said telephone instrument to its off hook state to blockapplication of detectable signals to said detecting means and therebyprevent the detection of a central office-transmitted signal of saidpre-selected frequency whenever said telephone instrument is in its offhook state..].
 15. In a telephone transmission system, a telephoneinstrument and a talking pair of conductors for connecting saidtelephone instrument to a central office, said conductors being adaptedto be connected to central office-located equipment that is capable ofbeing operated by transference of said telephone instrument to its offhook state, and a telephone loop testing circuit connected to saidconductors remotely from said central office and comprising means fordetecting a central office-transmitted signal of pre-selected frequency,switch means connected between said conductors in parallel with saidtelephone instrument, said switch means being capable of beingconditioned to complete a current conducting path that bridges saidconductors in parallel with said telephone instrument, means renderedeffective by said detecting means upon the detection of said centraloffice-transmitted signal for .[.conditioning.]. .Iadd.conducting.Iaddend.said switch means to complete said current conducting path,said current conditioning path being effective upon being completed tocause operation of said equipment as if said telephone instrument weretransferred to its off hook state and without disconnecting saidtelephone instrument from said central office.[...]..Iadd., said switchmeans being connected in parallel circuit relationship with saiddetecting means, and said loop testing circuit further comprising aunidirectional current-conducting device in series with said detectingmeans for blocking the application of detectable signals to saiddetecting means whenever said telephone instrument is in its off hookstate to thereby prevent the detection of a signal of said pre-selectedfrequency whenever said telephone instrument is in its off hook state..Iaddend. .[.16. The telephone transmission system defined in claim 15wherein said switch means is connected in parallel circuit relationshipwith said detecting means..]. .[.17. The telephone transmission systemdefined in claim 16 wherein said loop testing circuit comprises aunidirectional current conducting device in series with said detectingmeans for blocking the application of detectable signals to saiddetecting means whenever said telephone instrument is in its off hookstate to thereby prevent the detection of a signal of said pre-selectedfrequency whenever said telephone instrument is in its off hookstate..]. .[.18. The telephone transmission system defined in claim 15wherein said means for conditioning said switch means to complete saidcurrent conducting path includes time delay means rendered effective bythe detection of said signal of pre-selected frequency for conditioningsaid switch means to disable said current conducting path uponexpiration of a pre-selected time following the detection of said signalof pre-selected frequency by said detecting means..]. .[.19. Thetelephone transmission system defined in claim 18 wherein said looptesting circuit further comprises means for blocking application of adetectable signal to said detecting means while said current conductingpath is completed..]. .[.20. A telephone loop tester comprisingdetecting means adapted to be connected to a central office by a talkingpair of conductors of a telephone transmission line in a telephonetransmission system that has a telephone instrument connected to saidcentral office by said transmission line, said detecting means beingeffective to detect a central office-transmitted signal of pre-selectedfrequency, and circuit means responsive to the detection of said signalby said detecting means for completing a current-conducting circuit paththat bridges said talking pair of conductors in parallel with saidtelephone instrument and remotely from said central office to provide anoff hook signal as if said telephone instrument were actuallytransferred to its off hook state without disconnecting said telephoneinstrument from said central office and while said telephone instrumentis in its on hook state..]. .Iadd.
 21. A telephone loop tester adaptedto be electrically connected to a transmission medium in a telephonesystem in which said transmission medium provides a two way transmissionpath between a subscriber's telephone and a central office and includestip and ring conductors connected to said telephone set, said telephoneloop tester comprising a signal detecting circuit for detecting apre-selected central office-originated signal which is fed to thedetector circuit from said transmission medium, means responsive to thedetection of said pre-selected signal by said detecting circuit forcompleting a current shunt across said ring and tip conductors withoutdisconnecting said telephone set from said central office and withouttransferring said telephone set to its off hook condition, said shuntupon being completed beng effective to cause a change in an electricalcondition which is detectable at said central office if saidtransmission medium is operable, and a unidirectional current-conductingdevice electrically connected to said detecting circuit for blockingapplication of detectable signals to said detecting circuit wheneversaid telephone set is in its off hook state. .Iaddend. .Iadd.
 22. Atelephone loop tester adapted to be electrically connected to a talkingpair of conductors in a telephone system in which the talking pair ofconductors is connected to a subscriber's telephone equipment and formsat least a part of a transmission facility that connects said telephoneequipment to a central office, said telephone loop tester comprising atuned detector for sensing a central office-originated test signal ofpre-selected frequency which is transmitted by said transmissionfacility to said tuned detector, means responsive to the sensing of saidtest signal by said tuned detector for completing a current-conductingpath across the conductors of said talking pair without transferringsaid telephone equipment to its off hook condition, saidcurrent-conducting path upon being completed being effective to cause achange in an electrical condition that is detectable at said centraloffice to provide an indication that said transmission facility isoperative apart from the subscriber's telephone equipment, and meansrendered effective by the completion of said current-conducting path forpreventing application of a detectable test signal to said detectorwhile said path is completed. .Iaddend..Iadd.
 23. A telephone looptester adapted to be electrically connected to a talking pair ofconductors in a telephone system in which the talking pair of conductorsis connected to a subscriber's telephone equipment and forms at least apart of a transmission facility that connects said telephone equipmentto a central office, said telephone loop tester comprising a tuneddetector for sensing a central office-originated test signal ofpre-selected frequency which is transmitted by said transmissionfacility to said tuned detector, means responsive to the sensing of saidtest signal by said tuned detector for completing a current-conductingpath across the conductors of said talking pair without transferringsaid telephone equipment to its off hook condition, saidcurrent-conducting path upon being completed being effective to cause achange in an electrical condition that is detectable at said centraloffice to provide an indication that said transmission facility isoperative apart from the subscriber's telephone equipment, and aunidirectional current-conducting device electrically connected to saiddetector for blocking application of detectable test signals to saiddetector whenever said telephone equipment is in its off hook state..Iaddend. .Iadd.
 24. The telephone loop tester defined in claim 23,wherein said unidirectional current-conducting device and said detectorare electrically connected in series, and wherein said means forcompleting said current-conducting path comprises switch means inparallel with said detector and said device. .Iaddend. .Iadd.
 25. Atelephone loop tester adapted to be electrically connected to a talkingpair of conductors in a telephone system in which the talking pair ofconductors is connected to a subscriber's telephone equipment and formsat least a part of a transmission facility that connects said telephoneequipment to a central office, said telephone loop tester comprising atuned detector for sensing a central office-originated test signal ofpre-selected frequency which is transmitted by said transmissionfacility to said tuned detector, means responsive to the sensing of saidtest signal by said tuned detector for completing a current-conductingpath across the conductors of said talking pair without transferringsaid telephone equipment to its off hook condition, saidcurrent-conducting path upon being completed being effective to cause achange in an electrical condition that is detectable at said centraloffice to provide an indication that said transmission facility isoperative apart from the subscriber's telephone equipment, and meansrendered effective by the transfer of said telephone equipment to theoff hook state to block application of the test signal to said detector..Iaddend. .Iadd.
 26. A telephone loop tester adapted to be electricallyconnected to a talking pair of conductors in a telephone system in whichthe talking pair of conductors is connected to a subscriber's telephoneequipment and forms at least a part of a transmission facility thatconnects said telephone equipment to a central office, said telephoneloop tester comprising a tuned detector for sensing a centraloffice-originated test signal of pre-selected frequency which istransmitted by said transmission facility to said tuned detector, meansresponsive to the sensing of said test signal by said tuned detector forcompleting a current-conducting path across the conductors of saidtalking pair without transferring said telephone equipment to its offhook condition, said current-conducting path upon being completed beingeffective to cause a change in an electrical condition that isdetectable at said central office to provide an indication that saidtransmission facility is operative apart from the subscriber's telephoneequipment, and means rendered effective by the transfer of saidtelephone equipment to the off hook state for preventing the completionof said current-conducting path by the transmission of said test signalas long as said telephone equipment is in the off hook state. .Iaddend..Iadd.
 27. The telephone loop tester defined in claim 26, comprising atime delay circuit responsive to the detection of said test signal bysaid detector for maintaining said current-conducting path completed fora pre-selected time interval following termination of the detected testsignal. .Iaddend. .Iadd.
 28. A telephone loop tester adapted to beelectrically connected to a talking pair of conductors in a telephonesystem in which the talking pair of conductors is connected to asubscriber's telephone equipment and forms at least a part of atransmission facility that connects said telephone equipment to acentral office, said telephone loop tester comprising afrequency-discriminating, tuned detector for sensing a test signal ofpre-selected frequency which is transmitted by way of said facility tosaid detector, means responsive to the sensing of said test signal bysaid detector for completing a circuit across the conductors of saidtalking pair to cause a change in a predetermined electrical conditionthat is detectable at the central office to provide an indication thatsaid facility is operative apart from said telephone equipment, andmeans rendered effective by the transfer of said telephone equipment tothe off hook state for preventing the completion of said circuit uponthe transmittal of said test signal as long as said telephone equipmentis in the off hook state. .Iaddend. .Iadd.
 29. The telephone loop testerdefined in claim 28 wherein said detector is a resonant circuit..Iaddend..Iadd.
 30. The telephone loop tester defined in claim 28comprising a time delay circuit responsive to the sensing of said testsignal by said detector for maintaining said current-conducting pathcompleted for a pre-selected time interval following termination of thesensed test signal. .Iaddend. .Iadd.
 31. A telephone loop testercomprising tuned detecting means adapted to be connected to a centraloffice by a talking pair of conductors of a telephone transmission linein a telephone system that has a telephone set connected to said centraloffice by said transmission line, said detecting means being operativeto detect a signal of pre-selected frequency that is transmitted by saidtalking pair of conductors from said central office, means responsive tothe detection of said signal of pre-selected frequency by said detectingmeans for electrically simulating an off hook condition of saidtelephone set remotely from said central office to provide an off hooksignal while said telephone set is in its on hook state and withoutdisconnecting said telephone set from said central office, said meansfor simulating said off hook condition of said telephone set comprisinga circuit rendered effective by said detecting means upon detecting saidsignal of pre-selected frequency for completing a current-conductingcircuit path that bridges said conductors in parallel relation with saidtelephone set, and a unidirectional current-conducting device connectedto said detecting means for blocking application of detectable signalsto said detecting means whenever said telephone set is in its off hookstate. .Iaddend..Iadd.
 32. In a telephone transmission system, atelephone instrument and a talking pair of conductors for connectingsaid telephone instrument to a central office, said conductors beingadapted to be connected to central office-located equipment that iscapable of being operated by transference of said telephone instrumentto its off hook state, and a telephone loop testing circuit connected tosaid conductors remotely from said central office and comprising meansfor detecting a central office-transmitted signal of pre-selectedfrequency, switch means connected between said conductors in parallelwith said telephone instrument, said switch means being capable of beingconditioned to complete a current conducting path that bridges saidconductors in parallel with said telephone instrument, means renderedeffective by said detecting means upon the detection of said centraloffice-transmitted signal for conditioning said switch means to completesaid current conducting path, said current conducting path beingeffective upon being completed to cause operation of said equipment asif said telephone instrument were transferred to its off hook state andwithout disconnecting said telephone instrument from said centraloffice, said means for conditioning said switch means to complete saidcurrent conducting path including time delay means rendered effective bythe detection of said signal of pre-selected frequency for conditioningsaid switch means to disable said current-conducting path uponexpiration of a pre-selected time following the detection of said signalof pre-selected frequency by said detecting means, and said loop testingcircuit further comprising means for blocking application of adetectable signal to said detecting means while said current-conductingpath is completed. .Iaddend.